1. Field of the Invention
This invention relates generally to the field of in-situ soil and groundwater remediation, more particularly to a method for remediation of soil and groundwater which has become contaminated with heavy metals, semi-metals, and cyanide, and most particularly to the remediation of soils and groundwater contaminated with hexavalent chromium.
2. Description of the Prior Art
Subterranean contamination of soil and groundwater resulting from leaking storage facilities or accidental or even purposeful discharge has become a problem in almost all industrialized areas of the planet. Industrialized society had historically stored, discharged and disposed of various hazardous substances and waste products to the soil and groundwater. However, in recent decades, society has recognized that discharged substances contaminate the soil and groundwater and can thereby cause severe health hazards and damage to the local environment. Discharged substances have consisted of both organic and inorganic materials. The inorganic toxic discharges have included chromium, hexavalent chromium and other heavy metals, and semi-metals including substances such as cadmium, lead, copper, nickel, arsenic and cyanides.
The growing concern for the protection of the environment has led federal, state and local governments to enact a series of laws and regulations placing strict standards on the permissible percentages of toxic substances in waste waters, solids and solid wastes. As a consequence of past discharges and subsequent contamination, individuals, companies and governments have been forced to expend a great deal of time and financial resources in remediation. The present invention is an improved and efficient method for chemical remediation of heavy metals, semi-metals and cyanide in contaminated sites, and more particularly, a method for the remediation of hexavalent chromium.
The related prior art may be broadly categorized as falling into three groups:
Methods to prevent contamination by removing contaminants from liquid waste effluents.
U.S. Pat. Nos. 3,896,209, 3,901,805, 4,321,149, 4,724,084, 4,770,773, 4,996,715, 5,308,501, 5,316,684 and 5,545,331 describe methods, reagents and apparatus for removing heavy metals and/or cyanides from liquid effluents.
Methods to decontaminate soils necessitating excavation of the contaminated sites.
U.S. Pat. Nos. 5,158,686, and 5,304,710 describe methods, reagents and apparatus for this purpose. Such methods are very costly compared to the method of the invention.
Methods for in-situ remediation of toxic wastes in soils and groundwater.
U.S. Pat. Nos. 4,749,491, 5,285,000, 5,286,141, 5,397,478, 5,520,483, 5,525,008, 5,562,588 and 5,611,642 are most relevant to the present invention.
Of the in-situ remediation methods, U.S. Pat. Nos. 4,749,491, 4,749,491, 5,285,000, 5,286,141, 5,520,483 are directed toward remediation of organic contaminants such as hydrocarbons and halogenated hydrocarbons. These disclosures have a different objective and describe methods differing from the method of the present invention.
Closer to the present invention are U.S. Pat. No. 5,525,008 and U.S. Pat. No. 5,611,642, both of which are expressly incorporated herein by reference. U.S. Pat. No. 5,525,008 discloses a general remediation method. U.S. Pat. No. 5,611,642 discloses specially designed injector wells appropriate for use in the present invention.
The remaining prior art patents deal with in-situ methods for remediation of heavy metals including hexavalent chromium but describe different methods and different reagents than the present invention. U.S. Pat. No. 5,397,478 and U.S. Pat. No. 5,562,588 are in-situ methods only in a limited sense. U.S. Pat. No. 5,397,478 describes a chemical method for in-situ treatment requiring mechanical mixing, e.g. rototilling of the soil with reagents. This method is feasible and cost effective only in the case where the contamination lies very close to the surface. Similarly, U.S. Pat. No. 5,562,588 describes a bioremediation method requiring mixing of the soil with bacteria and nutrients. It has the same disadvantages as U.S. Pat. No. 5,397,478 and/or methods requiring excavation of the site.
U.S. Pat. No. 5,285,000 describes a two-solution injection method. First a reducing agent is injected into the soil, followed by a drying step and then injection of a gelling agent. In this method, the soil volume treated becomes impermeable to liquid flow shortly after injection of the gelling solution. Consequently, injection wells must be very closely spaced to achieve uniform saturation of the entire soil volume before permeability declines. This can be quite costly and completeness and uniformity of treatment cannot be assured. Normal flow of groundwater and surface water in and near the treated site is also disrupted.
A need exists for an in-situ method for chromium and other heavy metal remediation that is economical, thorough, uniform and verifiable. Claim 1 of U.S. Pat. No. 5,525,008 describes a method for the in-situ remediation of a contaminated underground area. This invention is an improvement of that method directed to remediation of chromium and other heavy metals, semi-metal and cyanide contamination.
The invention is an in-situ technology for removing metals from groundwater and permanently immobilizing the metals in an unsoluble form in the soil. The method comprises the steps of determining an effective amount and concentration of a first reactive solution comprising ferrous sulfate and an acid selected from the group consisting of sulfuric acid and phosphoric acid required to decomplex chromium and other heavy metals, semi-metals and cyanide from mineral surfaces. For most heavy metals, the oxidized state is the least soluble. However for chromium the valence state must be reduced whereas for most other heavy metals such as cadmium or arsenic the valence state must be increased. When hexavalent chromium is the contaminant, the addition of the first solution reduces it to the trivalent chromium which is thereby precipitated. Much less ferrous sulfate is preferred to treat other heavy metals.
An effective amount and concentration of a second reactive solution comprising hydrogen peroxide and an acid selected from the group consisting of sulphuric acid and phosphoric acid required to destroy organic ligands and enhance decomplexation; injecting the first reactive solution and subsequently the second reactive solution at an effective pressure and at a flow rate in excess of a sustainable yield of the amount of said first reactive solution and second reactive solution into one or more injectors that are inserted into the ground, sealed and positioned so as to assure liquid flow and dispersion of the reactive solutions through the contaminated area; and allowing said first and second reactive solutions to flow through the contaminated area, thereby reacting chemically with the contaminants contained therein. Preferably, the injection of the second reactive solution is pulsed.
When treating non-chromium contaminants, the iron concentration in the first treating solution may be reduced to 50 to 1000 mg/L and the peroxide concentration in the second solution increased to 1-25%.
It is preferred that the pH of the first reactive solution is in a range from about 3 to about 5 and the pH of the second reactive solution is in a range from about 3 to about 7. Most preferably the pH of the first reactive solution is in a range from about 3.5 to 4.5 and the pH of the second reactive solution is 5 to 7 when treating chromium. Normally the pH of solution 2 will range from 3 to 7. However if the solution is used to treat groundwater at a pH of 5 to 7, the treating solution can be at 5 to 7.
Advantageously, the present invention converts metal and semi-metal contaminants such as hexavalent chromium, cadmium, lead, copper, nickel and arsenic, as well as cyanides into environmentally inert substances without producing any collateral contamination. The inventive method effectively treats both soil and groundwater, regardless of whether the source of contamination is situated above or below ground level. Further, the invention does not disrupt the long-term normal flow of groundwater or surface water in or near the contaminated site and is applicable for both short and long-term treatment of a contaminated site.